This invention relates to multicolor heat transfer decals particularly as used to decorate plastic articles.
In order to produce heat transfers that give high quality images and show clear and brilliant colors, a screen process (e.g. silk screen) was originally used. The screen process enabled a printer to formulate inks that were compatible with the various plastics to be decorated since there were virtually no restrictions on the solvents that could be used to dissolve the resins that were needed to make the inks. Screen process inks are conventionally made using slow drying solvents such as white spirit, high boiling aromatics, ketones and glycol ethers.
To satisfy a larger volume market for heat transfers that could not be met by the comparatively slow silk screen process, various methods were devised using a gravure printing process. Gravure printing has the advantage that almost any solvent system can be used due to the solvent impervious etched metal rollers that are used to carry out gravure printing. For example, very fast evaporating solvents such as toluene, methylethyl-ketone and N-propyl acetate are used.
Many of the gravure heat transfer systems employ an adhesive layer or a wax to bond conventional printing inks down onto the final receptor surface or carrier film. U.S. Pat. Nos. 2,862,332 and 2,989,413 disclose examples of such a wax bonding system. The wax acts as a release system and as a bonding agent.
Other systems have a release layer similar to that used in the foil stamping industry. Such release layers are commonly mixtures of waxes and a high molecular weight acrylic. The inks associated with such systems are based on resins that are only soluble in very active solvents.
There are serious disadvantages in using either silk screen process or gravure process. The screen process is very slow and requires new screens each time a particular design is run since the screens are very susceptible to damage. With such processes, very fine color process printing beyond 60-80 lines per inch is difficult thereby seriously limiting the quality of work that can be produced. The nature of the processes preclude close register of prined material.
Since heat transfers are generally required in a roll or web format, screen printing machines having the ability to print on web form have been developed. However, they are very costly and require highly trained operating personnel.
The gravure process is ideal for producing high quality heat transfers. Thermoplastic inks printed over release coated film give the finest print quality. However, this too is a very costly process. The printing equipment is very expensive and the costs of preparing gravure cylinders is extremely high since highly skilled personnel are necessary for their manufacture. A gravure processing plant has a high capital cost further escalating the cost.
Alternative processes have been proposed. Various early heat transfers were printed using rubber plates on conventional flexographic equipment. However, the rubber plates give very poor quality reproduction and the inks used are not thermoplastic. Also, a secondary adhesive coating must be applied to give adhesion to the articles being decorated. The colors are very weak and tend to be transparent and have very poor resistance to abrasion. Consequently, this early process was discontinued.
In recent times the invention of plates made from photopolymer resins such as Dupont Cyrel.RTM. resin and or BASF Nyloprint.RTM. resin plates has resulted in a high quality plate that can give resolution and definition to letterpress printing much closer to that achieved by the gravure process. The solvent system that can be used with these "plastic" plates, however, is still similar to that used with the older rubber printing plates although the plastic plates will tolerate a higher percentage of more active solvent in the inks. Commonly used solvents therefore consist mainly of ethanol or isopropanol with additions in the order of 10% of propyl acetate, nitropropane and glycol ethers.